Previously, sol-gel methods have been developed to fabricate thin films, fibres, monoliths, spherical particles and various other structures prepared by coating structures with a gel film layer. Furthermore, micro- and nanotubes have been produced by covering structures with a gel film layer. Micro- and nanotubes have been fabricated from various oxide materials such as: ZnO, TiO2, SnO2, Pb3O4, CeO2, V2O3, NiO, rare earth metal oxides, SiO2 and Na2W2O7. Oxide micro- and nanotubes are principally prepared by coating a suitable structure (fibre or membrane) with a sol layer which is followed by gelation, thermal treatment and removal of the template.
A method is known for the manufacturing of tubular structures where mechanical stresses are induced due to the difference of material composition of the multiple-layered film structure deposited on a substrate. Such methods require specific deposition chambers such as vapour-phase deposition, molecular epitaxy (Prinz, V. Ya., Seleznev, V. A., Gutakovsky, A. K., Chehovckiy, A. V., Preobrazhenskii, V. V., Putyato, M. A., Gavrilova, T. A., Free-standing and overgrown InGaAs/GaAs nanotubes, nanohelices and their arrays., Physica E, 6 (2000)) or in case of polymeric materials dip-coating or spin-coating techniques are utilized (Luchnikov, V., Sydorenko, O., Stamm, M., Self-Rolled and Composite Polymer/Metal Micro- and Nanotubes with Patterned Inner Walls, Adv. Mater, 17 (2005))).
Afterwards, the films are cut into suitable size employing methods that require specific apparatus such as electron beam-, ion beam- or optical lithography. The film is released from the substrate by selective etching and the induced stress gradient causes the film to form a tubular structure. This method has been applied for producing micro- and nanotubes of various semi-conducting materials such as SiGe/Si, InGaAs/GaAs (Prinz, et al, 2000), In/Al/GaAs/InAs (Prinz, et al, 2000), InGaP, SiGe/Si/Cr, SiGe/Si/SixNy/Cr.
Titanium oxide nanotubes have been fabricated with a method described in the work of T, Hiramatsu, M., Hoson, A., Sekino, T., Niihara, K., Formation of titanium oxide nanotube. Langmuir, 14(12) (1998). Suitable crystal oxide materials or materials produced via the sol-gel route are employed as starting materials for this method. The starting material powder is treated with a base (e.g. NaOH) solution and then with an acidic (e.g. HCl) solution. The washing results in TiO2 nanotubes. Such TiO2 materials have the advantage over bulk TiO2 that they have a higher photocatalytic activity and therefore such materials are employed for purification of the environment from toxic gases, decomposition of carboxylic acids or production of hydrogen. As a biomaterial it is applicable in implants as it is compatible with the tissues of living organisms. In addition such materials are potentially applicable in solid electrolytes and fuel elements.
Comparable solutions to the current invention are the United State patents U.S. Pat. No. 6,537,517 (Crystalline titania having nanotube crystal shape and process for producing the same) and U.S. Pat. No. 6,027,775 (Crystalline titania and process for producing the same), which describe the processing of crystal titanium oxide powders with a basic solution that results in the formation of nanometre sized sheets that self-similarly form tubular structures.
The European patent EP1557396A2 can be considered the closest solution to the current invention. A multiple layered polymer structure is transformed on a surface and with the addition of solvent to one layer a change of volume induced. An intrinsic gradient of some physical-chemical or chemical property induces a volume change gradient in the material which causes the rolling of the film structure.
Similarly, microtubes of semi-conducting materials are fabricated. The film structures are deposited of materials that intrinsically exhibit stress at the common interface (V. Y. Prinz, V. A. Seleznev, A. K. Gutakovsky, A. V. Chehovskiy, V. V. Preobrazhenskii, M. A. Putyato, T. A. Gavrilova, “Free-standing and overgrown InGaAs/GaAs nanotubes, nanohelices and their arrays,” Physica E 6, 828-831 (2000).). The key stage of the described methods is the removal of the film structure by selectively etching the specific layer composed of another material from between the substrate and the film.
Another similar solution is the method for preparing a sol film on the surface of water. A selfstanding gel film is obtained that does not self-similarly form a tubular structure (M. Yamame, S. Shibata, A. Yasumori, T. Yano, S. Uchihiro, “Thick silicate glass film by an interfacial polymerization,” J. Sol-Gel Sci. Technol. 2, 457-460 (1994).). By employing a non-template method the fracturing of the materials due to contraction at gelation is avoided.
Although the field of application of oxide materials is extremely wide and metal oxides such as ZrO2, HfO2 and TiO2 are produced yearly in tonnes, the shortcoming of the known inventions is the complex and costly technological process. The utmost deficiency of solutions for oxide materials in certain fields (e.g. catalysis, thermo-isolation at high temperatures or high pressures) is the extremely high cost of the materials and products made thereof.